Photosynthesis

• Photosynthesis is a physico-chemical process in green plants where they use light energy to drive the synthesis of organic compounds
• Green plants make their own food through photosynthesis and are called autotrophs, while organisms depending on green plants for food are heterotrophs
• Photosynthesis is important because it is the primary source of all food on earth and responsible for the release of oxygen into the atmosphere by green plants
• Chlorophyll, light, and CO2 are required for photosynthesis to occur
• Photosynthesis takes place in the green leaves of plants and other green parts of plants
• Chloroplasts in mesophyll cells align themselves along the walls to get the optimum quantity of incident light
• Chloroplasts have a membranous system consisting of grana, stroma lamellae, and matrix stroma

  • Grana trap light energy and synthesize ATP and NADPH
  • Stroma enzymatic reactions synthesize sugar, which forms starch
  • Light reactions are directly light-driven, while dark reactions are dependent on the products of light reactions
• There are four pigments involved in photosynthesis:

  • Chlorophyll a (bright or blue green)
  • Chlorophyll b (yellow green)
  • Xanthophylls (yellow)
  • Carotenoids (yellow to yellow-orange)
• Pigments have the ability to absorb light at specific wavelengths
• Chlorophyll a is the most abundant plant pigment in the world
• Chlorophyll a shows maximum absorption at blue and red regions of the spectrum
• Chlorophyll a is the chief pigment associated with photosynthesis
• Accessory pigments like chlorophyll b, xanthophylls, and carotenoids absorb light and transfer energy to chlorophyll a
• Light reactions in photosynthesis include light absorption, water splitting, oxygen release, and formation of ATP and NADPH
• Photosystem I (PS I) and Photosystem II (PS II) are two discrete photochemical light harvesting complexes
• Photosystem I has a reaction centre chlorophyll a called P700, while Photosystem II has a reaction centre chlorophyll a called P680
• Electron transport in photosystem II absorbs 680 nm wavelength of red light, causing electrons to become excited and jump into an orbit farther from the atomic nucleus
• Electrons are passed through an electron transport chain to photosystem I, where they are transferred to NADP+ to form NADPH+H+
• The transfer of electrons from PS II to PS I, and the subsequent reduction of NADP+ to NADPH+H+, is called the Z scheme
• Splitting of water in PS II provides electrons to replace those removed from photosystem I, resulting in the production of oxygen
• Non-cyclic photophosphorylation occurs when both PS II and PS I work in series, synthesizing ATP and NADPH+H+
• Cyclic photophosphorylation occurs when only PS I is functional, resulting in the synthesis of ATP but not NADPH+H+
• ATP synthesis in chloroplasts is linked to the development of a proton gradient across the thylakoid membrane through the chemiosmotic hypothesis
• The breakdown of the proton gradient leads to the synthesis of ATP through the ATP synthase enzyme
• Chemiosmosis requires:

  • A membrane
  • A proton pump
  • A proton gradient
  • ATP synthase
• Energy is used to pump protons across a membrane to create a high concentration of protons within the thylakoid lumen
• ATP synthase has a channel that allows diffusion of protons back across the membrane, releasing energy to activate the ATP synthase enzyme
• ATP and NADPH produced by the movement of electrons are used in the biosynthetic phase of photosynthesis
• The biosynthetic phase does not directly depend on the presence of light but on the products of the light reaction: ATP, NADPH, CO2, and H2O
• The first product of CO2 fixation in the biosynthetic phase is a 3-carbon organic acid called 3-phosphoglyceric acid (PGA)
• The Calvin Cycle:

  • Carboxylation: CO2 is fixed into a stable organic intermediate by RuBP carboxylase, resulting in the formation of two molecules of 3-PGA
  • Reduction: Series of reactions lead to the formation of glucose, requiring 2 molecules of ATP and 2 of NADPH per CO2 molecule fixed
  • Regeneration: RuBP acceptor molecule is regenerated, requiring one ATP for phosphorylation to form RuBP
• For every CO2 molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required
• To make one molecule of glucose, 6 turns of the Calvin cycle are required
• C4 plants:

  • Have C4 oxaloacetic acid as the first CO2 fixation product but use the Calvin cycle as the main biosynthetic pathway
  • Have special leaf anatomy, tolerate higher temperatures, respond to high light intensities, lack photorespiration, and have greater biomass productivity
• C4 plants have 'Kranz' anatomy with bundle sheath cells around vascular bundles
• The Hatch and Slack Pathway in C4 plants involves PEP carboxylase fixing CO2 into a 3-carbon molecule, which is transported to bundle sheath cells for breakdown and release of CO2
• Photorespiration:

  • RuBisCO enzyme can bind to both CO2 and O2
  • In C3 plants, some O2 binds to RuBisCO, decreasing CO2 fixation and leading to photorespiration
• C3 Plants
• Cell type in which the Calvin cycle takes place: Mesophyll
• Cell type in which the initial carboxylation reaction occurs: Mesophyll